An apparatus using an inkjet head is known as being used for printing characters, letters, marks or the like on the surface of a tablet. For the inkjet head, in order to properly maintain the discharge amount, it is necessary to keep the water head difference between the height of the ink liquid level in the ink tank and the height of the nozzle surface of the inkjet head within an acceptable range.
For example, in a technique of performing printing on a printing sheet, an acceptable range of the liquid level of ink in an ink tank (sub tank) is set in advance, and the sensor is arranged at the upper limit position and the lower limit position of the acceptable range. When the liquid level deviates from the lower limit of the acceptable range, the ink is supplied up to the upper limit of the acceptable range.
In the case where such a printing technique is applied to printing on a printing sheet or film formation on a large substrate such as a liquid crystal substrate, a large amount of ink is discharged (consumed) at one time. For this reason, the acceptable range needs to be set wide enough.
Incidentally, the water head difference inevitably differs greatly between when the liquid level is at the upper limit position of the acceptable range and when the liquid level is at the lower limit position. This has little influence on printing on a printing object such as a printing sheet, a large substrate, and the like.
However, in the field of tablet printing, a character, a letter, a mark or the like having a size of about 1 mm is printed on tablets with a diameter of, for example, about 5 mm to 8 mm. When printing was carried out on such small printing objects by using the printing technique as described above, blur or ink bleeding occurred, and printing was not properly performed on the tablets.